A reflective liquid crystal display device comprises a pair of polarizers, a reflective layer disposed on the external side of one of the polarizers, and a TN (twisted nematic) liquid crystal device or an STN (super-twisted nematic) liquid crystal device sandwiched between the pair of polarizers. However, this type of reflective liquid crystal display device has the problem that not only display brightness is low, because the reflective layer is located outside the glass substrate, shadows appear in the display due to parallax. This type of reflective liquid crystal display is called the external-reflector-type reflective display with the reflective layer provided below the bottom substrate when viewed from the top, i.e., the viewer side of the display. Here, a description will be given of how shadows appear in the display. In a liquid crystal display device having such an external reflector type structure, light incident from an obliquely upward direction is reflected by the reflective layer, and exits the liquid crystal display device in an obliquely upward direction. Such obliquely incident light emerges from the liquid crystal display device at a position different from the position where the light entered. This positional difference increases as the distance between the light incidence surface and the reflection surface in the liquid crystal display device increases. On the other hand, electrodes formed on the bottom substrate of the liquid crystal display device have a certain degree of reflectivity, and the obliquely incident light reflected by the electrodes and the obliquely incident light reflected by the reflector disposed behind the substrate are observed as being displaced in position; this displacement shows up as a shadow.
In view of this, there is proposed a reflective liquid crystal display device of an internal reflector type in which a reflective layer is formed within a liquid crystal device and which can produce a display by using a single polarizer. This reflective liquid crystal display device of an internal reflector type, which uses only one polarizer, can achieve higher brightness than the conventional reflective liquid crystal display device that uses two polarizers.
Further, by forming the reflective layer within the liquid crystal device, it also becomes possible to solve the problem of display shadows occurring due to the thickness of the glass substrate. That is, in the internal reflective display device with the reflective layer provided above the bottom substrate, as the reflective layer is located near or in the same position as the electrodes, the above-described displacement is virtually eliminated, and a shadow, if any, is negligibly small.
For the reflective layer, a transflective layer, that is produced by a thin aluminum film formed extremely thin with a thickness of 0.01 μm to 0.03 μm by evaporation or sputtering, can be used. There has also been developed a transflective liquid crystal display device of internal reflector type which uses, as the reflective layer, a transflective layer having openings formed therein, one for each pixel, by photo etching, and which produces a display by using backlighting when the ambient light is low as in the nighttime.
FIG. 15 shows a cross-sectional view of one example of a transflective liquid crystal display device (refer to JP-A-2003-172925).
In the transflective liquid crystal display device shown in FIG. 15, a scattering layer 15, a liquid crystal polymer phase retardation plate 12, and a first polarizer 11 are provided above a liquid crystal device 30, and a quarter-wave length phase retardation plate 14, a second polarizer 17, and a backlight 16 are provided below the liquid crystal device 30.
The liquid crystal device 30 comprises: a first substrate 1 made of a 0.5-mm thick glass plate on which are formed a 0.15-μm thick transflective layer 7 made of aluminum, a 2-μm thick protective layer 8 made of an acrylic-based material, and 0.2-μm thick first electrodes 3 as transparent electrodes made of ITO; a second substrate 2 made of a 0.5-mm thick glass plate on which 0.2-μm thick second electrodes 4 made of ITO are formed; a seal member 5 for bonding the first and second substrates 1 and 2 together; and a liquid crystal layer 6 formed from a left-handed 240° twisted nematic liquid crystal sandwiched between the first and second substrates 1 and 2.
The liquid crystal polymer phase retardation plate 12 has a right-handed twist, the twist angle being 180°, and the Δnd value, i.e., the product of the birefringence difference Δn of the liquid crystal polymer and the thickness d of the liquid crystal polymer layer, is 0.73 μm. The retardation value of the quarter-wave length phase retardation plate 14 is 0.14 μm. This retardation value corresponds to about quarter of the green light wavelength of 0.55 μm. The birefringence difference Δn of the nematic liquid crystal is 0.15, and the cell gap d between the first substrate 1 and the second substrate 2 is 5.6 μm; therefore, the Δnd value that represents the birefringence of the liquid crystal device 30 is 0.84 μm (=0.15×5.6).
In the internal-reflector-type transflective liquid crystal display device that uses the above-described liquid crystal polymer phase retardation plate 12, the twist angle, the Δnd value, and the orientation angle of the liquid crystal polymer phase retardation plate 12 are optimized, and a difference is provided between the twist angle of the liquid crystal device and the twist angle of the liquid crystal polymer phase retardation plate and between the Δnd value of the liquid crystal device and the Δnd value of the liquid crystal polymer so that the combined birefringence of the liquid crystal polymer phase retardation plate 12 and the liquid crystal layer 6 becomes approximately equal to quarter wavelength. When no voltage is applied in the reflective display mode (OFF state), the incident light is circularly polarized at the transflective layer 7, and the circularly polarized light once again passes through the liquid crystal layer 6 and the liquid crystal polymer phase retardation plate 12 to form linearly polarized light oriented at right angles to the incident light; therefore, the liquid crystal display appears black. When a voltage is applied in the reflective display mode (ON state), the Δnd value of the liquid crystal device decreases, and the combined birefringence of the liquid crystal polymer phase retardation plate 12 and the liquid crystal layer 6 becomes nearly zero. Therefore, the incident light passed through the first polarizer arrives at the transflective layer 7 without changing its linear polarization state, and returns unchanged to the first polarizer 11 as the linearly polarized light oriented parallel to the incident light; therefore, the liquid crystal display appears white. In this case, a reflective display of good contrast can be achieved.
If the scattering layer 15 is formed using a forward scattering material having a property that allows the incident light to easily pass through it (mainly in the traveling direction of the incident light) and that exhibits good scattering performance to scatter the emerging light reflected by the reflector (i.e., to scatter the light mainly in the forward direction), the emerging light is scattered in all directions, and the visibility of the display increases. Instead of providing the scattering layer 15, the transflective layer 7 itself may be roughened to provide a scattering surface, or the protective layer 8 may be formed to have a scattering material and used as a scattering protective film.
When no voltage is applied in the transmissive display mode (OFF state), light emitted from the backlight 16 is circularly polarized by passing through the second polarizer 17 and the quarter-wave length phase retardation plate 14. The combined birefringence of the liquid crystal polymer phase retardation plate 12 and the liquid crystal device 30 is approximately equal to quarter wavelength. The quarter-wave length phase retardation plate 14 is oriented at such an angle that the phase retardation value of the liquid crystal polymer phase retardation plate 12, the liquid crystal layer 6, and the quarter-wave length phase retardation plate 14 combined becomes nearly zero. The light is converted back to the linearly polarized light when passed through the liquid crystal polymer phase retardation plate 12, but since the transmission axis of the second polarizer 17 is oriented at right angles to the transmission axis of the first polarizer 11, the transmitted light is absorbed by the first polarizer 11, and thus the liquid crystal display appears black. When a voltage is applied in the transmissive display mode (ON state), the combined birefringence of the liquid crystal polymer phase retardation plate 12 and the liquid crystal layer 6 becomes nearly zero; as a result, the light emitted from the backlight 16 and circularly polarized by the quarter-wave length phase retardation plate 14 arrives unchanged at the first polarizer 11, and thus the liquid crystal display appears white.
However, the circularly polarized light emerging from the quarter-wave length phase retardation plate 14 is difficult to compensate over the entire visible wavelength range by using both the liquid crystal device 30 and the liquid crystal polymer phase retardation plate 12. Accordingly, in the transmissive display mode, it has been difficult to achieve perfect black, and hence the first problem that sufficient contrast cannot be obtained. Further, of the light emitted from the backlight 16, the light reflected by the unopened portions of the transflective layer 7 and returned in the direction of the backlight is rotated 90° by passing through the quarter-wave length phase retardation plate 14 twice, and is therefore absorbed by the second polarizer. Hence, there has also been the second problem that the display becomes dark.
To address the second problem, there is proposed a transflective liquid crystal display device in which the transflective layer is provided with a patterning phase retardation layer.
FIG. 16 shows one example of such a transflective liquid crystal display device in which the transflective layer is provided with a patterning phase retardation layer (refer to JP-A-2003-279957).
In the liquid crystal display device shown in FIG. 16, the patterning phase retardation layer 9 whose phase retardation value is approximately equal to quarter wavelength is provided on the unopened portions of the transflective layer 7. Another feature of the liquid crystal display device shown in FIG. 16 is that the phase retardation plate between the first substrate 1 and the second polarizer 17 and the phase retardation plate or the liquid crystal polymer phase retardation plate between the second substrate 2 and the first polarizer 11 are eliminated.
In the liquid crystal display device shown in FIG. 16, the birefringence of the liquid crystal device 31, in the absence of an applied voltage (OFF state), is set approximately equal to one-quarter wavelength, while in the presence of an applied voltage (ON state), the liquid crystal molecules stand up perfectly and the birefringence thus becomes nearly zero. In the ON state of the reflective display mode, the combined phase retardation value of the liquid crystal layer 6 and the patterning phase retardation layer 9 is equal to quarter wavelength. Accordingly, the incident light entering the liquid crystal display device is circularly polarized at the transflective layer 7, and the reflected light passes through the patterning phase retardation layer 9 and the liquid crystal layer 6, and is thus converted into linearly polarized light oriented at right angles to the incident light, so that the light is absorbed by the first polarizer 11, and thus the liquid crystal display appears black. On the other hand, in the OFF state of the reflective display mode, the combined phase retardation value of the liquid crystal layer and the patterning phase retardation layer is zero. Accordingly, the incident light returns to the first polarizer 11 without changing its polarization state, so that the light passes through the first polarizer, and thus the liquid crystal display appears white. That is, the liquid crystal display device shown in FIG. 16 has a normally white mode in which the display is white in the absence of an applied voltage.
In the ON state of the transmissive display mode, as the patterning phase retardation layer 9 is not formed on the openings of the transflective layer 7, the light emitted from the backlight 16 and passed through the second polarizer 17 directly enters the liquid crystal layer 6. Here, as the transmission axis of the second polarizer 17 is oriented at right angles to the transmission axis of the first polarizer 11, the birefringence of the liquid crystal layer 6 is zero, and the incident light is absorbed by the first polarizer 11; therefore, the liquid crystal display appears black. In the OFF state of the transmissive display mode, since the birefringence of the liquid crystal layer is approximately equal to quarter wavelength, the light passed through the liquid crystal layer 6 is circularly polarized and passes through the first polarizer 11. Therefore, the liquid crystal display appears white.
On the other hand, of the light passed through the second polarizer 17, the light reflected by the unopened portions of the transflective layer 7 returns toward the backlight without changing its polarization state, because the quarter-wave length phase retardation plate 14 provided in the internal-reflector-type transflective liquid crystal display device shown in FIG. 15 is not provided here; therefore, the light once again passes through the second polarizer 17 and returns to the backlight 16. As the light returned to the backlight 16 is reflected by the backlight toward the liquid crystal display device, a bright transmissive display can be obtained.
FIG. 17 shows an example in which, in order to improve the contrast of the reflective display, a half-wave length phase retardation plate 19 is provided between the second substrate 2 and the first polarizer 11 in the liquid crystal display device shown in FIG. 16 (refer to JP-A-2003-279956).
In the liquid crystal display device shown in FIG. 17, the retardation value of the half-wave length phase retardation plate 19 is 0.28 μm which is about half of a green light wavelength of 0.55 μm, and the retardation value of the patterning phase retardation layer 9 is 0.14 μm which is approximately equal to quarter wavelength; here, the axes of the half-wave length phase retardation plate 19 and the patterning phase retardation layer 9 are oriented at about 60° to each other. With the phase retardation and the chromatic dispersion characteristics of the patterning phase retardation layer and the half-wave length phase retardation plate combined, the so-called broadband circular polarization phase retardation plate is constructed in which the retardation value for the shorter wavelength is smaller than the retardation value for the longer wavelength. Accordingly, in the ON state of the reflective display mode, the birefringence of the liquid crystal device 32 is zero, so that the incident light entering through the first polarizer 11 is converted into circularly polarized light over the entire visible wavelength range by the half-wave length phase retardation plate 19 and the patterning phase retardation layer 9, and reflected by the unopened portions of the transflective layer 7. The light reflected by the unopened portions of the transflective layer 7 once again passes through the patterning phase retardation layer 9 and the half-wave length phase retardation plate 19 and is thus converted into linearly polarized light oriented at right angles to the incident light over the entire visible wavelength range; therefore, the light is absorbed by the first polarizer 11. Accordingly, the liquid crystal display device shown in FIG. 17 can achieve a higher-contrast reflective display than the liquid crystal display device shown in FIG. 16.